BASIC OPERATION AND FEATURES
SX TRANSISTOR CONTROL
Page 3
SPEED
TORQUE
ARMATURE CURRENT
Figure 2
In the separately excited motor, the motor is operated as a
fixed field shunt motor in the normal running range.
However, when additional torque is required, for example,
to climb non-level terrain, such as ramps and the like, the
field current is increased to provide the higher level of
torque. In most cases, the armature to field ampere turn
ratio can be very similar to that of a comparable size series
motor (Figure 3.)
STARTING
CURRENT
FULL
LOAD CURRENT
NO LOAD CURRENT
SPEED
In a shunt motor, the variation of speed from no load to
normal full load on level ground is less than 10%. For this
reason, shunt motors are considered to be constant speed
motors (Figure 2).
STARTING
CURRENT
The level of sophistication in the controllability of traction
motors has changed greatly over the past several years.
Vehicle manufacturers and users are continuing to expect
more value and flexibility in electric vehicle motor and
control systems as they are applied today. In order to
respond to these market demands, traction system
designers have been forced to develop new approaches to
reduce cost and improve functions and features of the
overall system. Development is being done in a multigenerational format that allows the market to take
advantage of today’s technology, while looking forward to
new advances on the horizon. GE has introduced a second
generation system using separately excited DC shunt
wound motors. The separately excited DC motor system
offers many of the features that are generally found on the
advanced AC systems. Historically, most electric vehicles
have relied on series motor designs because of their ability
to produce very high levels of torque at low speeds. But, as
the demand for high efficiency systems increases, i.e.,
systems that are more closely applied to customers’
specific torque requirements, shunt motors are now often
being considered over series motors. In most applications,
by independently controlling the field and armature
currents in the separately excited motor, the best attributes
of both the series and the shunt wound motors can be
combined.
FULL
LOAD CURRENT
Section 1.1 Motor Characteristics
providing the greater torque needed to drive the increased
mechanical load. If the mechanical load is decreased, the
process reverses. The motor speed and the back EMF
increase, while the armature current and the torque
developed decrease. Thus, whenever the load changes, the
speed changes also, until the motor is again in electrical
balance.
NO LOAD CURRENT
Section 1. INTRODUCTION
SPEED
TORQUE
In a shunt motor, the field is connected directly across the
voltage source and is therefore independent of variations in
load and armature current. If field strength is held constant,
the torque developed will vary directly with the armature
current. If the mechanical load on the motor increases, the
motor slows down, reducing the back EMF (which depends
on the speed, as well as the constant field strength). The
reduced back EMF allows the armature current to increase,
STARTING
CURRENT
NO LOAD CURRENT
As shown in the typical performance curves of Figure 1, the
high torque at low speed characteristic of the series motor
is evident.
FULL
LOAD CURRENT
ARMATURE CURRENT
Figure 1
TORQUE
ARMATURE CURRENT
Figure 3
Aside from the constant horsepower characteristics
described above, there are many other features that
provide increased performance and lower cost. The
October 2005
BASIC OPERATION AND FEATURES
SX TRANSISTOR CONTROL
Page 4
following description provides a brief introduction to some
of these features.
Section 1. 2 Solid-State Reversing
and armature, the motor performance curve can be
maximized through proper control application.
Section 1. 4 More Features with Fewer Components
The direction of armature rotation on a shunt motor is
determined by the direction in which current flows through
the field windings. Because of the shunt motor field,
typically only requires about 10% of the armature current at
full torque, it is normally cost effective to replace the
double-pole, double-throw reversing contactor with a low
power transistor H-Bridge circuit (Figure 4).
LINE POS
FUSE
Q3
Q5
A1 +
ARM
CAP
Q2
F1
F2
A2 Q4
Q1
NEG
Figure 4
By energizing the transistors in pairs, current can be made
to flow in either direction in the field. The field and
armature control circuits typically operate at 12KHZ to
15KHZ, a frequency range normally above human hearing.
This high frequency, coupled with the elimination of
directional contactors, provides for very quiet vehicle
operation.
The line contactor is normally the only contactor required
for the shunt motor traction circuit. This contactor is used
for both pre-charge of the line capacitors and for
emergency shut down of the motor circuit, in case of
problems that would cause a full motor torque condition.
The line can be energized and de-energized by the various
logic combinations of the vehicle, i.e. activate on key, seat
or start switch closure, and de-energize on time out of idle
vehicle. Again, these options add to the quiet operation of
the vehicle.
Section 1. 3 Flexible System Application
Because the shunt motor controller has the ability to
control both the armature and field circuits independently,
the system can normally be adjusted for maximum system
efficiencies at certain operating parameters. Generally
speaking, with the ability to independently control the field
Q6
Field weakening with a series wound motor is
accomplished by placing a resistor in parallel with the field
winding of the motor. Bypassing some of the current
flowing in the field into the resistor causes the field current
to be less, or weakened. With the field weakened, the motor
speed will increase, giving the effect of “overdrive”. To
change the “overdrive speed”, it is necessary to change
the resistor value. In a separately excited motor,
independent control of the field current provides for
infinite adjustments of “overdrive” levels, between the
motor base speed and maximum weak field. The
desirability of this feature is enhanced by the
elimination of the contactor and resistor required for
field weakening with a series motor.
With a separately excited motor, overhauling speed
limit, or downhill speed, will also be more constant. By
its nature, the shunt motor will try to maintain a
constant speed downhill. This characteristic can be
enhanced by increasing the field strength with the
control. Overhauling load control works in just the
opposite way of field weakening, as armature rotation
slows with the increase of current in the field. An
extension of this feature is a zero-speed detect feature
which prevents the vehicle from free-wheeling down an
incline, should the operator neglect to set the brake.
Regenerative braking (braking energy returned to the
battery) may be accomplished completely with solid-state
technology. The main advantage of regenerative braking is
increased motor life. Motor current is reduced by 50% or
better during braking while maintaining the same braking
torque as electrical braking with a diode clamp around the
armature. The lower current translates into longer brush
life and reduced motor heating. Solid state regenerative
braking also eliminates a power diode, current sensor and
contactor from the circuit.
For GE, the future is now, as we make available a new
generation of electric traction motor systems for electric
vehicles having separately excited DC shunt motors and
controls. Features that were once thought to be only
available on future AC or brushless DC technology vehicles
systems are now achievable and affordable.
Section 2. FEATURES OF SX FAMILY OF TRANSISTOR
MOTOR CONTROLLERS
Section 2.1 Performance
Section 2.1.1 Oscillator Card Features
October 2005